Electrochemical and spectroelectrochemical characterisation of cyano and trifluoromethyl substituted polypyridines and their transition metal complexes

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Abstract

This thesis is concerned with the electrochemical and spectroelectrochemical
characterisation of cyano (CN) and trifluoromethyl (CF3) substituted polypyridine
ligands and their metal complexes. The ligands investigated were X-CN-py (X = 3, 4
and 5, py = pyridine), X,X´-(CN)2-bpy) (X,X´ = 3,3´, 4,4´ and 5,5´ bpy = 2,2´-
bipyridine) and X,X´-(CF3)2-bpy (X,X´ = 3,3´, 4,4´ and 5,5´). The Pt(II) complexes of
the X-CN-py and X,X´-(CN)2-bpy ligands were studied along with the Fe(II)
complexes of the X,X´-(CN)2-bpy and X,X´-(CF3)2-bpy ligands.
Electrochemical studies of the X-CN-py ligands indicated that 2-CN-py and 4-CN-py
have one reversible 1e- reduction and 3-CN-py has one quasi-reversible reduction.
4-CN-py is more easily reduced than 2-CN-py. EPR experiments on [2-CN-py]1- and
[4-CN-py]1- combined with DFT calculations have indicated that the semi-occupied
molecular orbital (SOMO) is delocalised over the entire molecule. The complex
[Pt(4-CN-py)2Cl2] undergoes two reversible 1e- reductions, attributed to the sequential
reduction of the two 4-CN-py ligands. [Pt(3-CN-py)2Cl2] was found to exhibit one
irreversible reduction.
The electrochemistry of the X,X´-(CN)2-bpy ligands indicated that 3,3´-(CN)2-bpy
and 5,5´-(CN)2-bpy have two reversible 1e- reductions while 4,4´-(CN)2-bpy has only
one reversible 1e- reduction. The reductions of 5,5´-(CN)2-bpy occur at significantly
less negative potentials than those in 3,3´ or 4,4´ analogue. In-situ UV/Vis/NIR and
EPR experiments on the X,X´-(CN)2-bpy ligands indicate that the reduction electron
is delocalised over both the CN-py rings with the SOMO being spread across the
entire molecule. [Pt(3,3´-(CN)2-bpy)Cl2] and [Pt(5,5´-(CN)2-bpy)Cl2] undergo two reversible 1e- reductions, attributed to the sequential reduction of the X,X´-(CN)2-bpy
ligand.
4,4´-(CF3)2-bpy and 5,5´-(CF3)2-bpy, whose X-ray crystallographically determined
structures are reported, both have one reversible 1e- reduction while 3,3´-(CF3)2-bpy
exhibits an irreversible reduction. Again the 5,5´ analogue is the most easily reduced.
Spectroelectrochemical results indicate that the reduction electron enters a SOMO that
is delocalised over both the CF3-py rings. Substitution in the 5,5´ positions is
determined to be electronically most significant.
Complexes of the general formula [Fe(II)(X2-bpy)3][BF4] (where X = CN or CF3 in
the 4,4´ or 5,5´ positions) exhibit three reversible ligand based reduction processes
and a metal based oxidation. UV/Vis/NIR and EPR studies have confirmed that the
reduction electron in each case enters a molecular orbital that is predominantly based
on a bpy ligand.
The redox potentials of 4-CN-py and 5,5´-(CN)2-bpy are solvent dependent with 1e-
reduction of 4-CN-py and the two 1e- reductions of 5,5´-(CN)2-bpy moving to less
negative potentials as the acceptor number (AN) of the solvent increases.
A computational model has been developed for the study of the electronic properties
of substituted bipyridines using DFT methods. This model has been used to aid the
analysis of the EPR and UV/Vis/NIR spectra of the X2-bpys studied.